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Accurate Characterization of SiO2 Thin Films Using Surface Acoustic Waves Matthias Knapp, Member, IEEE, Alexey M. Lomonosov, Paul Warkentin, Philipp M. Jäger, Werner Ruile, Hans-Peter Kirschner, Matthias Honal, Ingo Bleyl, Andreas P. Mayer, Member, IEEE, and Leonhard M. Reindl, Member, IEEE Abstract—We have investigated the acoustic properties of silicon dioxide thin films. Therefore, we determined the phase velocity dispersion of LiNbO 3 substrate covered with SiO 2 deposited by a plasma enhanced chemical vapor deposition and a physical vapor deposition (PVD) process using differential delay lines and laser ultrasonic method. The density ρ and the elastic constants (c 11 and c 44) can be extracted by fitting corresponding finite element simulations to the phase velocities within an accuracy of at least ±4%. Additionally, we propose two methods to improve the accuracy of the phase velocity determination by dealing with film thickness variation of the PVD process.
I. Introduction
R
ecently, the demand for excellent filter performance has led to rapid development of layered systems for surface acoustic wave (SAW) filters. Especially, the improvement in temperature stability has become a topic of interest. There are different strategies to improve the temperature stability of SAW devices [1]. The approach that is studied most extensively uses a silicon dioxide (SiO2) overlayer [2], [3]. This amorphous layer becomes mechanically stiff with increasing temperature. It therefore has a positive temperature coefficient of velocity that is correlated with its temperature coefficient of elasticity. This feature can be used to compensate the softening of the substrate with increasing temperature. However, an additional layer changes the characteristics of the SAW device significantly. Therefore, an accurate knowledge of the acoustic properties of the dielectric layer is essential for filter design and optimization. Thus, we will determine the elastic constants of the SiO2 thin film using two different evaluation methods.
Manuscript received December 4, 2014; accepted February 13, 2015. M. Knapp, P. Jäger, W. Ruile, H.-P. Kirschner, M. Honal, and I. Bleyl are with EPCOS AG, a Group Company of TDK Corporation, 81671 Munich, Germany (e-mail: [email protected]). A. M. Lomonosov, P. Warkentin, and A. P. Mayer are with Faculty Betriebswirtschaft + Wirtschaftsingenieurwesen, Hochschule Offenburg, University of Applied Sciences, 77723 Gengenbach, Germany. A. M. Lomonosov is also with General Physics Institute, Russian Academy of Sciences, 119991 Moscow, Russia. M. Knapp and L. M. Reindl are with Department of Microsystems Engineering, University of Freiburg, 79110 Freiburg, Germany. DOI http://dx.doi.org/10.1109/TUFFC.2014.006921
II. Differential Delay-Lines A. Evaluation Method Simulation methods for SAW devices are based on the fundamental linear wave equations for the materials in multilayer structures [4]. Therefore, it is necessary to have a precisely determined set of material constants of the materials involved in the layer stack to be able to achieve good agreement between simulation and the experimental results. However, thin film characteristics are usually not well known due to difficulties in measurement and accurate evaluation and its dependence on process conditions. So far, different approaches are mentioned in literature determining the thin film properties using nanoindentation [5], laser ultrasonics method [6], brillouin scattering [7], picosecond ultrasonic measurement [8], or differential delay lines [9]. The advantage of the differential delay line approach is that all deteriorating effects coming from, for example, metal electrodes for wave excitation of layer geometry, are eliminated because the only difference between the delay lines is the shift Δs in Fig. 1. The idea of this method is to determine the phase velocity dispersion of the layer stack and afterwards extract the elastic constants by fitting corresponding finite element simulations to the phase velocities. In a first step the group velocity vgr of the SAW is measured.
v gr =
∆s ∆s = . (1) ∆τ τ1 − τ 2
The difference in delay line length Δs is exactly known from the test structure geometry. The difference in time delay Δτ is calculated from the magnitude of the measured delay line signals in time domain. This information provides the approximate number of wavelengths fitting into the distance Δs. The exact number of wavelengths can be obtained, if additionally the phase information ∆ϕ of the impulse responses is used.
∆ϕ( fm ) = m ⋅ 2π = k( fm ) ⋅ ∆s, (2) v ph ( fm ) =
2π ⋅ fm 2π ⋅ fm ⋅ ∆s . (3) = k( fm ) 2π ⋅ m
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knapp et al.: accurate characterization of sio2 thin films using surface acoustic waves
Fig. 1. Schematic picture of a differential delay line.
Thereby, fm is the frequency where Δs corresponds to a phase difference which is an integer multiple m of phase turns 2π. More details can be found in a previous publication [10]. The measured phase velocities and the simulation of a Rayleigh wave on LiNbO3 128° rot. YX without additional layers can be seen in Fig. 2. Several different transducer geometries on one wafer were used to measure the phase velocity of the Rayleigh wave at different frequencies. The linear regression of the measured phase velocities at large wavelengths (i.e., frequency f →0) is in excellent agreement with the simulation within Δvph = 25 ppm. The values published in [11] and [12] were used in our FEM-simulations. For higher frequencies the measurements show a tiny dispersion on LiNbO3 of 480 ppm/GHz, which might be due to a damaged layer on the surface. Because this effect is small, it has not been considered in the simulation of the free surface. The evaluation method and the constants for the substrate have thus been tested with high accuracy and can now be used as a starting point for evaluating layered systems. B. Film Preparation and Measurement SiO2 films of two different thicknesses of about 600 and 1400 nm were deposited on LiNbO3 128° rot. YX and LiNbO3 64° rot. YX substrates by a plasma enhanced chemical vapor deposition (PECVD) and a physical vapor deposition (PVD) process. These two substrate cuts were used to measure a Rayleigh wave and a shear wave, which is necessary to determine the material constants of the films. The accuracy of the evaluated data using differential delay line method depends on the prevention of disturbing reflected waves on the edges of the films. Therefore, the deposited SiO2 films cover the whole structures shown in Fig. 1. The refractive index of the SiO2 films at a wavelength of λ = 632.8 nm is measured using spectral ellipsometry.
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Fig. 2. Comparison between measured and simulated phase velocities of a Rayleigh wave on LiNbO3 128° rot. YX.
The measured values can be seen in Table I. Additionally, a value published in the literature is listed which shows excellent agreement with our measurement. The layer thickness of the films was measured by spectral ellipsometry. The layer thickness variation across the wafer of the PECVD process is displayed in Fig. 3 and the variation of the PVD process can be seen in Fig. 4. The relative layer thickness variation Δh/h is calculated as h − hmin ∆h . (4) = max h hmean
Thereby, hmax is the maximal determined layer thickness, hmin is the minimal determined layer thickness, and hmean is the mean layer thickness. The thickness variation of the PECVD process is Δh/hPECVD = 0.3% and is within the measurement accuracy of the profilometer. The variation of the PVD process is significantly higher with Δh/hPVD = 3.4% in this experiment. Fig. 5 shows the phase velocities of the Rayleigh wave on LiNbO3 128° rot. YX covered with different SiO2 layers, determined with the differential delay line method described above. The phase velocity in the PVD SiO2/ LiNbO3 system is higher than for the PECVD SiO2/LiNbO3 system for a given relative layer thickness h/λ. The standard deviation for the phase velocity in the case of the PECVD SiO2/LiNbO3 system is only Δvph(PECVD) = 1.6 m/s, whereas for the PVD SiO2/LiNbO3 system we lose an order of magnitude in accuracy and get Δvph(PECVD) = 17.3 m/s. This large deviation might be due to a violation of some implicit assumptions of the differential delay line method, which are TABLE I. Refractive Index of SiO2 Thin Films. Process PECVD PVD Naturally grown
n
Reference
1.465 1.472 1.47
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Fig. 3. Layer thickness variation of SiO2 PECVD across the wafer.
• The interdigital transducers are identical in geometry as well as their electroacoustic properties. • The film thicknesses in the delay lines, which are compared, are the same. • The inner properties (density and elastic constants) of the thin film are the same. • The measurement conditions are identical using the same equipment, calibration, and setup. In the following we will assume that only the condition of identical film thickness is violated. Therefore, the layered structure including the PVD SiO2 shows a significantly higher phase velocity deviation than the PECVD SiO2-based layer stack. We propose two methods to correct this thickness variation to improve the phase velocity accuracy. C. Experimental Improvement of On-Wafer Phase Velocity Deviation A proposal to improve the phase velocity determination is to decrease the layer thickness variation. There-
Fig. 4. Layer thickness variation of SiO2 PVD across the wafer.
Fig. 5. Comparison of measured phase velocities of LiNbO3 128° rot. YX substrate covered with PECVD and PVD SiO2.
fore, we were etching the wafer locally using a mechanical and chemical ion beam etching process. This ion beam is bundled to produce clusters. Accelerated by electric fields, these clusters were sputtered locally on the wafer to remove material from the surface layer. Thus, we were able to improve the layer thickness uniformity significantly (Fig. 6) to ΔhPVD = 0.9%. The comparison of the phase velocity determined before and after etching of the Rayleigh wave on LiNbO3 128° rot. YX covered with PVD SiO2 can be seen in Fig. 7. The standard deviation could be improved to Δvph = 4.6 m/s, which is significantly less than the evaluated values reported in Section II-A. However, trimming of the wafer leads to a slight velocity reduction as can be seen in the h/λ region of 10% to 25% in Fig. 8. Therefore, we propose a surface layer of about 20 nm thickness caused by the etching process to describe the phase velocity behavior in simulation. The material constants of this surface layer can be evaluated using the optimization method explained in [13]. The sur-
Fig. 6. Layer thickness variation of SiO2 PVD across the wafer after etching.
knapp et al.: accurate characterization of sio2 thin films using surface acoustic waves
Fig. 7. Comparison of phase velocity before and after trimming of a Rayleigh wave on LiNbO3 128° rot. YX covered with PVD SiO2.
face layer has reduced stiffness values of about 30% and about 10% reduced density compared with the values of the SiO2 layer with an assumed thickness of 20 nm due to the impact depth of the cluster beam. Additionally, we were tempering the wafer at 270°C for about 2 hours to confirm that we produced a system with stable behavior. A new phase velocity determination led to no significant velocity shift (black dots in Fig. 8), which proves that we processed a stable layered system. D. Expansion of Differential Delay Line Evaluation Method on Varying Layer Thicknesses A different possible evaluation improvement uses signal processing to consider the layer thickness deviation across the wafer. Therefore, a reference thickness is defined. Delay lines whose thickness differs from this reference thickness are modulated in their frequency response using appropriate signal processing methods. We correlated the measured layer thickness variation across the wafer with the measured delay time for every delay line. In Fig. 9 an example of a correlation can be seen for an IDT geometry of λ = 6 µm. The measured values show a linear correlation between layer thickness and delay time. The slope, calculated by linear regression, represents the delay time offset produced by layer thickness difference. This slope is used to correct the thickness calculation. We define a reference thickness for every set of differential delay lines. As soon as there is a difference between measured layer thickness and reference thickness, the discrepancy is corrected by shifting the impulse response by t0. t0 represents the delay time offset. The impulse response is changed according to [14].
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Fig. 8. Comparison of phase velocity before and after etching and after tempering.
t0). In the spectral domain the multiplication of the signal U(f) with the factor e−i ωt0 results in an appropriate signal modulation. After this thickness correction for every pair of delay lines, the phase velocity can be recalculated. The comparison between the uncorrected phase velocity dispersion and the corrected phase velocity dispersion of the PVD process can be seen in Fig. 10. The phase velocity deviation is significantly reduced. The accuracy of the evaluation was improved by more than a factor 4 from Δvph,PVD = 17.3 m/s down to Δvph,PVD = 3.9 m/s. E. Material Constants Determination Simulations of the layered system were fitted to the corresponding measured phase velocities by optimizing the density ρ and the elastic constants c11 and c44 of the thin film. Therefore, different methods are suitable to simulate the free multilayer surface, such as Green’s function
u(t − t0 ) F e−i 2πt0 fU ( f ). (5) ↔
Thereby, the symbol F represents the Fourier transform. ↔ The time shift u(t) by the time t0 leads to a signal u(t −
Fig. 9. Correlation of layer thickness and measured delay time at λ = 6 µm.
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Fig. 10. Comparison of uncorrected and corrected phase velocities of the PVD SiO2 process.
Fig. 11. Comparison of simulated and measured phase velocities of SAWs on LiNbO3 128° rot. YX covered with PECVD and PVD SiO2.
or finite elements (FEM). We decided to use FEM for convenience. The constants of the substrate were taken from [11] and [12] and assumed to be accurate. Phase velocities of the layered system are simulated using FEM with randomly changing material constants of the film until they fit the corresponding measured phase velocities. The comparison between the measured phase velocities of LiNbO3 128° rot. YX covered with PECVD SiO2 and PVD SiO2, the FEM simulations can be seen in Fig. 11, and the comparison on LiNbO3 64° rot. YX is displayed in Fig. 12. The corresponding density ρ and elastic constants c11 and c44 of the PECVD and PVD SiO2 films are listed in Table II. The measured density was verified by weighing of the wafer before and after deposition of the SiO2 layer. The evaluated value using differential delay lines (Table II) is in excellent agreement with the value by weighing (ρ = 2174 ± 148 kg/m3). The rather large error using weighing arises from inaccuracy determining the volume of the film. Additionally, literature values of a PECVD and a PVD SiO2 were added to Table II. The method for how to determine the error of the material constants is explained in a different publication [15].
contact-free excitation and detection of the SAWs. Additionally, this technique provides a broadband wave excitation [17]. A. Film Preparation Silicon dioxide of about 4 µm thickness was deposited on a LiNbO3 substrate. Therefore, plasma enhanced chemical vapor deposition method (PECVD) was used due to its uniform growth on the substrate (see Section II-B). The layer thickness was determined using spectral ellipsometry. Additionally, a 30-nm-thick aluminum metal film was deposited on top of the SiO2 film. B. Excitation and Detection of SAWs A Nd:YAG laser at a wavelength of 1064 nm with 1 ns pulse duration is used to excite the SAWs. The laser pulse is sharply focused onto a line using a cylindrical
III. Laser Ultrasonic Measurement A different approach to determine the elastic constants of thin films uses laser induced SAWs. This technique has already been extensively used. An example is the determination of the dependency of the Young’s modulus of microcrystalline CVD diamond films on deposition conditions [6] or the observation of SAW dispersion generated by gradients in the mechanical and elastic properties of millimeter-thick microcrystalline diamond plates [16]. We used this approach to confirm the dispersion curve and thus the elastic constants determined by the differential delay-line technique. The advantage of using laser ultrasonics for determination of elastic constants is an efficient
Fig. 12. Comparison of simulated and measured phase velocities of SAWS on LiNbO3 64° rot. YX covered with PECVD and PVD SiO2.
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TABLE II. Comparison of Material Tensor Data of SiO2 Thin Films. c11 [GPa]
c44 [GPa]
ρ [kg/m3]
67.9 ± 1.4 84.5 ± 2.5 75.0 78.0
25.2 ± 0.6 31.7 ± 1.3 22.5 29.0
2143.6 ± 38.3 2347.4 ± 74.1 2185.0 2275.0
lens to achieve a well-defined propagation direction and to prevent diffraction [18]. The pump pulses are absorbed in the thin metal film within a depth of tens of nanometers. Thus, two counterpropagating strain pulses are launched by thermoelastic emission. These propagating surface perturbations were registered at different distances from the source. For this purpose, the continuous wave laser probe-beam deflection method is used [19] to monitor the transient slope of the SAW. A schematic picture of the measurement setup can be seen in Fig. 13. C. Measurement Results The propagation distance was scanned by 30 steps of 100 µm. This determines the spatial resolution of 3 mm. At every measured distance the Fourier transform of the SAW wave form was calculated, and for each Fourier component the phase ϕ was determined as a function of propagation distance x. Each of these phase curves was fitted by a linear function:
ϕ(ω) =
ω ⋅ x + ϕ0 (ω), (6) v ph (ω)
where ω = 2π∙f is the angular frequency and vph the phase velocity. The accuracy of the analysis can be improved significantly by using many steps. The evaluation method is explained in more detail in a previous publication [17]. We were measuring the Rayleigh wave mode, which propagates on LiNbO3 128° rot. YX. Additionally, we were measuring another Rayleigh wave mode by rotating the wafer by 90°. These two modes carry the information on the mechanical properties of the PECVD layer and were thus used for its evaluation. We were using the material constants determined in Section II-E to simulate
Fig. 13. Schematic diagram of a nanosecond pump-probe setup for generating SAW pulses with a pulsed laser and laser probing deflection with a position-sensitive detector.
Process
Reference
PECVD PVD PECVD PVD
This work This work [20] [21]
the phase velocities determined using laser ultrasound. The comparison between the measured phase velocities on LiNbO3 128° rot. YX covered with PECVD SiO2 and the simulated values can be seen in Fig. 14. The accuracy of the determined material constants using differential delay lines exceeds the accuracy using laser ultrasonics at the moment. However, the laser ultrasonics method has not been optimized yet. IV. Summary and Discussion We have investigated the acoustic properties of PECVD and PVD SiO2 thin films on LiNbO3 substrates by a differential delay line method. The density ρ and elastic constants c11 and c44 of the SiO2 films were determined by fitting FEM simulations to the measured phase velocities within an accuracy of at least ±4%. We found that the phase velocity of PVD SiO2 is consistently higher compared with PECVD SiO2. The PVD deposition process shows a significant variation in thickness across the wafer, in contrast to the PECVD process. Therefore, the delay lines used in the differential delay line evaluation method have different thicknesses, although they are close to each other. Thus, we introduced an experimental method to decrease the thickness variation using local etching of the wafer. This improved the accuracy of the phase velocity determination by a factor of 4. However, this method slightly changed the acoustic properties of the SiO2 layer.
Fig. 14. Comparison of measured and simulated phase velocities of LiNbO3 128° rot. YX and LiNbO3 128° rot. Y 90° rot. X covered with about 4 µm of PECVD SiO2.
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Therefore, we propose another method to improve the phase velocity determination accuracy, introducing a reference thickness for both delay lines used. We shift the corresponding time delays using the correlation between layer thickness variation and measured delay time. Thus we were able to reduce the scatter in phase velocity by at least a factor of 4. This method is favorable because it does not change the material characteristics. Thus the differential delay line evaluation method was successfully extended for application on layer stacks with nonuniform thicknesses. Within the accuracy of the determined phase velocities, the elastic constants of the SiO2 films do not vary across the wafer and between different wafers, if the same deposition method was applied. Additionally, we were measuring the SAW dispersion relation for two different propagation directions on LiNbO3 covered with PECVD SiO2 using laser-based method. Excellent agreement was achieved between the measurements of the phase velocities by the differential delay line method and the laser probe analysis. Acknowledgments The authors thank W. Gawlik and H. Öztürk for providing accurate wafer prober metrology, G. Scheinbacher for SiO2 layer deposition, and G. Grünauer for providing assistance in optimization. References [1] K. Hashimoto, M. Kadota, T. Nakao, M. Ueda, and M. Miura, “Recent development of temperature compensated SAW devices,” in Proc. IEEE Ultrasonics Symp., 2011, pp. 79–86. [2] T. E. Parker and H. Wichansky, “Temperature-compensated surface-acoustic-wave devices with SiO2 film overlays,” J. Appl. Phys., vol. 50, no. 3, pp. 1360–1369, 1979. [3] S. Matsuda, M. Miura, T. Matsuda, M. Ueda, Y. Satoh, and K. Hashimoto, “Investigation of SiO2 film properties for zero temperature coefficient of frequency SAW devices,” in IEEE Ultrasonics Symp., 2010, pp. 633–636. [4] V. I. Cherednick and M. Y. Dvoesherstov, “Surface and bulk acoustic waves in multilayer structures,” in Waves in Fluids and Solids, Rijeka, Croatia: InTech, 2011, pp. 69–102. [5] T. Malkow, I. Arce-Garcia, A. Kolitsch, D. Schneider, S. J. Bull, and T. F. Page, “Mechanical properties and characterisation of very thin CNx films synthesised by IBAD,” Diam. Relat. Mater., vol. 10, no. 12, pp. 2199–2211, 2001. [6] R. Kuschnereit, P. Hess, D. Albert, and W. Kulisch, “Density and elastic constants of hot-filament-deposited polycrystalline diamond films: Methane concentration dependence,” Thin Solid Films, vol. 312, pp. 66–72, 1998. [7] M. W. Elmiger, J. Henz, H. V. Kãnel, M. Ospelt, and P. Wachter, “Characterization of crystal surfaces, thin films and superlattices by brillouin scattering from surface acoustic modes,” Surf. Interface Anal., vol. 14, no. 1–2, pp. 18–22, 1989. [8] L. L. Chapelon, D. Neira, J. Torres, J. Vitiello, J. C. Royer, D. Barbier, F. Naudin, G. Tas, P. Mukundhan, and J. Clerico, “Measuring the Young’s modulus of ultralow-k materials with the nondestructive picosecond ultrasonic method,” J. Microelectron. Eng., vol. 83, no. 11–12, pp. 2346–2350, 2006. [9] R. Thomas, T. W. Johannes, W. Ruile, and R. Weigel, “Determination of phase velocity and attenuation of surface acoustic waves with improved accuracy,” in Proc. IEEE Ultrasonics Symp., 1998, pp. 277–282.
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[10] M. Knapp, P. Jäger, G. Grünauer, G. Scheinbacher, I. Bleyl, and L. M. Reindl, “Accurate determination of thin film properties using SAW differential delay lines,” in IEEE Int. Ultrasonics Symp., 2013, pp. 1704–1707. [11] G. Kovacs, M. Anhorn, H. E. Engan, G. Visintini, and C. C. W. Ruppel, “Improved material constants for LiNbO3 and LiTaO3,” in IEEE Ultrasonics Symp. Proc., 1990, pp. 435–438. [12] R. T. Smith and F. S. Welsh, “Temperature dependence of the elastic, piezoelectric and dielectric constants of lithium tantalate and lithium niobate,” J. Appl. Phys., vol. 42, no. 6, pp. 2219–2226, 1971. [13] G. Grünauer, M. Mayer, M. Knapp, P. Jäger, T. Ebner, K. Wagner, and H. J. Pesch, “Derivation of accurate tensor data of materials in SAW devices by solving a parameter identification problem using an enhanced eigenvalue analysis of an infinite array model,” in IEEE Int. Ultrasonics Symp., 2013, pp. 1700–1703. [14] H. Marko, Methoden der Systemtheorie. Berlin, Germany: Springer Verlag, 1977. [15] M. Knapp, P. Jäger, W. Ruile, I. Bleyl, and L. M. Reindl, “A refined method to determine the elastic constants of SiO2 thin films,” in IEEE Ultrasonics Symp., 2014, pp. 277–280. [16] G. Lehmann, M. Schreck, L. Hou, J. Lambers, and P. Hess, “Dispersion of surface acoustic waves in polycrystalline diamond plates,” Diam. Relat. Mater., vol. 10, no. 3–7, pp. 686–692, 2001. [17] Z. H. Shen, A. M. Lomonosov, P. Hess, M. Fischer, S. Gsell, and M. Schreck, “Multimode photoacoustic method for the evaluation of mechanical properties of heteroepitaxial diamond layers,” J. Appl. Phys., vol. 108, no. 8, art. no. 083524, 2010. [18] P. Hess, A. M. Lomonosov, and A. P. Mayer, “Laser-based linear and nonlinear guided elastic waves at surfaces (2d) and wedges (1d),” Ultrasonics, vol. 54, no. 1, pp. 39–55, 2014. [19] H. Coufal, K. Meyer, R. K. Grygier, P. Hess, and A. Neubrand, “Precision measurement of the surface acoustic wave velocity on silicon single crystals using optical excitation and detection,” J. Acoust. Soc. Am., vol. 95, no. 2, p. 1158–1160, 1994. [20] M. Tomar, V. Gupta, and K. Shreenivas, “Temperature coefficient of elastic constants of SiO2 over-layer on LiNbO3 for temperature stable SAW device,” J. Phys. D, vol. 36, no. 15, pp. 1773–1776, 2003. [21] F. S. Hickernell, H. D. Knuth, R. C. Dablemont, and T. S. Hickernell, “The surface acoustic wave propagation characteristics of 64° Y-X LiNbO3 and 36° Y-X LiTaO3 substrates with thin-film SiO2,” in IEEE Ultrasonics Symp., 1995, pp. 345–348.
Matthias Knapp studied physics at the Ruprecht-Karls University of Heidelberg, Germany, where he received his Dipl.-Phys. degree in 2011. Since then, he has been pursuing his Ph.D. degree, studying the characterization of functional layers in Surface Acoustic Wave (SAW) devices as an external Ph.D. student at the Laboratory for Electrical Instrumentation of Prof. L. M. Reindl in the Department of Microsystems Engineering (IMTEK), University of Freiburg, Germany. Since 2012 he has been an external employee at the EPCOS AG, which is a Group Company of TDK Corporation since 2008, in Munich, Germany, as a member in the Research and Development (R&D) Department. His primary research topics are signal processing and the characterization of layered systems for SAW devices. Matthias Knapp is a member of the IEEE.
Alexey M. Lomonosov studied at the Physics Faculty of Moscow State University, where he received his Master degree in 1984. He received the degree of Dr. Rer. Nat. at the University of Heidelberg, Germany, in 2002. In 1984 he joined the Prokhorov General Physics Institute of the Russian Academy of Sciences in Moscow, where he currently holds a position as Senior Research Fellow. His main field of research is laser physics, in particular laser ultrasound, which he pursues at his home institution and during research stays at the University of Heidelberg, Germany, the University of Le Mans, France, as a visiting professor at Nanjing University of Science and Technology, China, and at the University of Le Mans, France. He developed
knapp et al.: accurate characterization of sio2 thin films using surface acoustic waves methods to demonstrate the propagation of solitary SAW pulses, the investigation of brittle fracture of solids by means of strongly nonlinear pulses and nondestructive evaluation of residual stresses in metals.
Paul Warkentin, born in 1983, studied electrical engineering at Hochschule Offenburg, University of Applied Sciences. He received his Bachelor of Engineering degree in mechatronics in 2009 and his Master of Engineering degree in electrical engineering in 2012. In the same year, he took up a position as assistant at Hochschule Offenburg, where he worked in research and teaching projects in the areas of robotics, vehicle dynamics, and laser ultrasound.
Philipp M. Jäger, born in 1980, studied engineering physics at the University of Applied Sciences Munich, Germany. He received his Dipl. Ing. (FH) degree in 2010. After graduating he joined the R&D department of the Systems, Acoustics, Waves Business Group of EPCOS AG, a TDK Group Company. Since that time he has been involved in material research for acoustic wave devices and the development and optimization of these.
Werner Ruile received the Dipl. Phys. degree from the Ludwig-Maximilians University, Munich, Germany, in 1984. He received the Dr. Ing. degree of the Technical University Munich, Germany, in 1994. He was with the Corporate Research and Development, Siemens AG, Munich from 1983 until 2000, where he developed the simulation tools for SAW devices based on the P-Matrix. He joined the SAW research and development activities of the EPCOS AG in 2000, which is a Group Company of TDK Corporation since 2008, and worked on power durability, loss reduction, temperature compensation and trimming techniques. He currently focuses on nonlinearities in SAW filters.
Hans-Peter Kirschner was born 1966 in Rosenheim, Germany. He received his first degree of electronic technician in telecommunication at Deutsche Telekom in 1987 and his second degree of Dipl.-Ing. in electrical engineering at Fachhochschule Lands- hut, Germany, in 1993. Since then he was employed at EPCOS AG Munich, Germany, which is a Group Company of TDK Corporation since 2008, where he took part in the development of various processes for production of SAW filter for instance anodic passivation and frequency trimming. His primary interests are material modifications and precision corrective etching of wafer surface with ion beam and gas cluster ion beam technology.
Matthias Honal, born 1970 in Munich, Germany, received the Dipl. Min. degree from LudwigMaximilians University (LMU), Munich Germany in 1994 and the Dr. Rer. Nat. degree from the Federal Institute of Technology (ETH) Zurich, Switzerland in 1997, both in the field of crystallography. In 1999 he was a member of the micro acoustics group of the Siemens Corporate Technology Department, Munich, Germany, where he was engaged in research and development on SAW
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devices mainly concentrating on substrate materials at high temperatures. After several academic positions at Australian National University (ANU) Canberra, Australia, LMU Munich, Germany and University of Augsburg, Germany in 2000 to 2003 he joined the materials analytics group of Siemens Corporate Technology Department, Munich. Since early 2010 he has been employed at EPCOS AG, Munich, Germany, which is a Group Company of TDK Corporation since 2008, where his primary research topics are on improvement of materials and processing technologies for SAW devices.
Ingo Bleyl was born in 1968 in Kulmbach, Germany. He received his diploma degree in Experimental Physics in 1995 and his Ph.D. in 1998 from the University Bayreuth, Germany. In 1999 he joined EPCOS AG, which has been a Group Company of TDK Corporation since 2008. Within the Systems, Acoustics, Waves Business Group, he is involved in material research and development of surface acoustic wave devices.
Andreas P. Mayer received his Dipl. Phys. degree in 1983 and his Dr. Rer. Nat. degree in 1985 at the University of Münster, Germany. After research work at the universities of Münster, Perugia, Edinburgh, and UC–Irvine on lattice dynamics, surface physics, and nonlinear optics, he was an assistant and Privatdozent at the Physics Department of the University of Regensburg, Germany (1991–2001). In 2002, he joint Siemens VDO as a development engineer and in 2008 he became a professor at HS Offenburg, University of Applied Sciences. His main research interests are in microacoustics, especially nonlinear effects on wave propagation. Andreas Mayer is a member of IEEE.
Leonhard M. Reindl received the Dipl. Phys. degree from the Technical University of Munich, Germany in 1985 and the Dr. Sc. Techn. degree from the University of Technology Vienna, Austria, in 1997. From 1985 to 1999 he was a member of the microacoustics group of the Siemens Corporate Technology department, Munich, Germany, where he was engaged in research and development on SAW convolvers, dispersive, and reflective delay lines. His primary interest was in the development and application of SAW ID-tag and wireless passive SAW sensor systems. In winter 1998–1999 and in summer 2000, he was a guest professor for spread spectrum technologies and sensor techniques at the University of Linz, Austria. From 1999-2003 he was university lecturer for communication and microwave techniques at the Institute of Electrical Information Technology, Clausthal University of Technology. In May 2003 he accepted a full professor position at the laboratory for electrical instrumentation at the Department of Microsystems Engineering (IMTEK), University of Freiburg.
IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control ,
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Accurate Characterization of SiO2 Thin Films Using Surface Acoustic Waves Matthias Knapp, Member, IEEE, Alexey M. Lomonosov, Paul Warkentin, Philipp M. Jäger, Werner Ruile, Hans-Peter Kirschner, Matthias Honal, Ingo Bleyl, Andreas P. Mayer, Member, IEEE, and Leonhard M. Reindl, Member, IEEE Abstract—We have investigated the acoustic properties of silicon dioxide thin films. Therefore, we determined the phase velocity dispersion of LiNbO 3 substrate covered with SiO 2 deposited by a plasma enhanced chemical vapor deposition and a physical vapor deposition (PVD) process using differential delay lines and laser ultrasonic method. The density ρ and the elastic constants (c 11 and c 44) can be extracted by fitting corresponding finite element simulations to the phase velocities within an accuracy of at least ±4%. Additionally, we propose two methods to improve the accuracy of the phase velocity determination by dealing with film thickness variation of the PVD process.
I. Introduction
R
ecently, the demand for excellent filter performance has led to rapid development of layered systems for surface acoustic wave (SAW) filters. Especially, the improvement in temperature stability has become a topic of interest. There are different strategies to improve the temperature stability of SAW devices [1]. The approach that is studied most extensively uses a silicon dioxide (SiO2) overlayer [2], [3]. This amorphous layer becomes mechanically stiff with increasing temperature. It therefore has a positive temperature coefficient of velocity that is correlated with its temperature coefficient of elasticity. This feature can be used to compensate the softening of the substrate with increasing temperature. However, an additional layer changes the characteristics of the SAW device significantly. Therefore, an accurate knowledge of the acoustic properties of the dielectric layer is essential for filter design and optimization. Thus, we will determine the elastic constants of the SiO2 thin film using two different evaluation methods.
Manuscript received December 4, 2014; accepted February 13, 2015. M. Knapp, P. Jäger, W. Ruile, H.-P. Kirschner, M. Honal, and I. Bleyl are with EPCOS AG, a Group Company of TDK Corporation, 81671 Munich, Germany (e-mail: [email protected]). A. M. Lomonosov, P. Warkentin, and A. P. Mayer are with Faculty Betriebswirtschaft + Wirtschaftsingenieurwesen, Hochschule Offenburg, University of Applied Sciences, 77723 Gengenbach, Germany. A. M. Lomonosov is also with General Physics Institute, Russian Academy of Sciences, 119991 Moscow, Russia. M. Knapp and L. M. Reindl are with Department of Microsystems Engineering, University of Freiburg, 79110 Freiburg, Germany. DOI http://dx.doi.org/10.1109/TUFFC.2014.006921
II. Differential Delay-Lines A. Evaluation Method Simulation methods for SAW devices are based on the fundamental linear wave equations for the materials in multilayer structures [4]. Therefore, it is necessary to have a precisely determined set of material constants of the materials involved in the layer stack to be able to achieve good agreement between simulation and the experimental results. However, thin film characteristics are usually not well known due to difficulties in measurement and accurate evaluation and its dependence on process conditions. So far, different approaches are mentioned in literature determining the thin film properties using nanoindentation [5], laser ultrasonics method [6], brillouin scattering [7], picosecond ultrasonic measurement [8], or differential delay lines [9]. The advantage of the differential delay line approach is that all deteriorating effects coming from, for example, metal electrodes for wave excitation of layer geometry, are eliminated because the only difference between the delay lines is the shift Δs in Fig. 1. The idea of this method is to determine the phase velocity dispersion of the layer stack and afterwards extract the elastic constants by fitting corresponding finite element simulations to the phase velocities. In a first step the group velocity vgr of the SAW is measured.
v gr =
∆s ∆s = . (1) ∆τ τ1 − τ 2
The difference in delay line length Δs is exactly known from the test structure geometry. The difference in time delay Δτ is calculated from the magnitude of the measured delay line signals in time domain. This information provides the approximate number of wavelengths fitting into the distance Δs. The exact number of wavelengths can be obtained, if additionally the phase information ∆ϕ of the impulse responses is used.
∆ϕ( fm ) = m ⋅ 2π = k( fm ) ⋅ ∆s, (2) v ph ( fm ) =
2π ⋅ fm 2π ⋅ fm ⋅ ∆s . (3) = k( fm ) 2π ⋅ m
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knapp et al.: accurate characterization of sio2 thin films using surface acoustic waves
Fig. 1. Schematic picture of a differential delay line.
Thereby, fm is the frequency where Δs corresponds to a phase difference which is an integer multiple m of phase turns 2π. More details can be found in a previous publication [10]. The measured phase velocities and the simulation of a Rayleigh wave on LiNbO3 128° rot. YX without additional layers can be seen in Fig. 2. Several different transducer geometries on one wafer were used to measure the phase velocity of the Rayleigh wave at different frequencies. The linear regression of the measured phase velocities at large wavelengths (i.e., frequency f →0) is in excellent agreement with the simulation within Δvph = 25 ppm. The values published in [11] and [12] were used in our FEM-simulations. For higher frequencies the measurements show a tiny dispersion on LiNbO3 of 480 ppm/GHz, which might be due to a damaged layer on the surface. Because this effect is small, it has not been considered in the simulation of the free surface. The evaluation method and the constants for the substrate have thus been tested with high accuracy and can now be used as a starting point for evaluating layered systems. B. Film Preparation and Measurement SiO2 films of two different thicknesses of about 600 and 1400 nm were deposited on LiNbO3 128° rot. YX and LiNbO3 64° rot. YX substrates by a plasma enhanced chemical vapor deposition (PECVD) and a physical vapor deposition (PVD) process. These two substrate cuts were used to measure a Rayleigh wave and a shear wave, which is necessary to determine the material constants of the films. The accuracy of the evaluated data using differential delay line method depends on the prevention of disturbing reflected waves on the edges of the films. Therefore, the deposited SiO2 films cover the whole structures shown in Fig. 1. The refractive index of the SiO2 films at a wavelength of λ = 632.8 nm is measured using spectral ellipsometry.
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Fig. 2. Comparison between measured and simulated phase velocities of a Rayleigh wave on LiNbO3 128° rot. YX.
The measured values can be seen in Table I. Additionally, a value published in the literature is listed which shows excellent agreement with our measurement. The layer thickness of the films was measured by spectral ellipsometry. The layer thickness variation across the wafer of the PECVD process is displayed in Fig. 3 and the variation of the PVD process can be seen in Fig. 4. The relative layer thickness variation Δh/h is calculated as h − hmin ∆h . (4) = max h hmean
Thereby, hmax is the maximal determined layer thickness, hmin is the minimal determined layer thickness, and hmean is the mean layer thickness. The thickness variation of the PECVD process is Δh/hPECVD = 0.3% and is within the measurement accuracy of the profilometer. The variation of the PVD process is significantly higher with Δh/hPVD = 3.4% in this experiment. Fig. 5 shows the phase velocities of the Rayleigh wave on LiNbO3 128° rot. YX covered with different SiO2 layers, determined with the differential delay line method described above. The phase velocity in the PVD SiO2/ LiNbO3 system is higher than for the PECVD SiO2/LiNbO3 system for a given relative layer thickness h/λ. The standard deviation for the phase velocity in the case of the PECVD SiO2/LiNbO3 system is only Δvph(PECVD) = 1.6 m/s, whereas for the PVD SiO2/LiNbO3 system we lose an order of magnitude in accuracy and get Δvph(PECVD) = 17.3 m/s. This large deviation might be due to a violation of some implicit assumptions of the differential delay line method, which are TABLE I. Refractive Index of SiO2 Thin Films. Process PECVD PVD Naturally grown
n
Reference
1.465 1.472 1.47
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Fig. 3. Layer thickness variation of SiO2 PECVD across the wafer.
• The interdigital transducers are identical in geometry as well as their electroacoustic properties. • The film thicknesses in the delay lines, which are compared, are the same. • The inner properties (density and elastic constants) of the thin film are the same. • The measurement conditions are identical using the same equipment, calibration, and setup. In the following we will assume that only the condition of identical film thickness is violated. Therefore, the layered structure including the PVD SiO2 shows a significantly higher phase velocity deviation than the PECVD SiO2-based layer stack. We propose two methods to correct this thickness variation to improve the phase velocity accuracy. C. Experimental Improvement of On-Wafer Phase Velocity Deviation A proposal to improve the phase velocity determination is to decrease the layer thickness variation. There-
Fig. 4. Layer thickness variation of SiO2 PVD across the wafer.
Fig. 5. Comparison of measured phase velocities of LiNbO3 128° rot. YX substrate covered with PECVD and PVD SiO2.
fore, we were etching the wafer locally using a mechanical and chemical ion beam etching process. This ion beam is bundled to produce clusters. Accelerated by electric fields, these clusters were sputtered locally on the wafer to remove material from the surface layer. Thus, we were able to improve the layer thickness uniformity significantly (Fig. 6) to ΔhPVD = 0.9%. The comparison of the phase velocity determined before and after etching of the Rayleigh wave on LiNbO3 128° rot. YX covered with PVD SiO2 can be seen in Fig. 7. The standard deviation could be improved to Δvph = 4.6 m/s, which is significantly less than the evaluated values reported in Section II-A. However, trimming of the wafer leads to a slight velocity reduction as can be seen in the h/λ region of 10% to 25% in Fig. 8. Therefore, we propose a surface layer of about 20 nm thickness caused by the etching process to describe the phase velocity behavior in simulation. The material constants of this surface layer can be evaluated using the optimization method explained in [13]. The sur-
Fig. 6. Layer thickness variation of SiO2 PVD across the wafer after etching.
knapp et al.: accurate characterization of sio2 thin films using surface acoustic waves
Fig. 7. Comparison of phase velocity before and after trimming of a Rayleigh wave on LiNbO3 128° rot. YX covered with PVD SiO2.
face layer has reduced stiffness values of about 30% and about 10% reduced density compared with the values of the SiO2 layer with an assumed thickness of 20 nm due to the impact depth of the cluster beam. Additionally, we were tempering the wafer at 270°C for about 2 hours to confirm that we produced a system with stable behavior. A new phase velocity determination led to no significant velocity shift (black dots in Fig. 8), which proves that we processed a stable layered system. D. Expansion of Differential Delay Line Evaluation Method on Varying Layer Thicknesses A different possible evaluation improvement uses signal processing to consider the layer thickness deviation across the wafer. Therefore, a reference thickness is defined. Delay lines whose thickness differs from this reference thickness are modulated in their frequency response using appropriate signal processing methods. We correlated the measured layer thickness variation across the wafer with the measured delay time for every delay line. In Fig. 9 an example of a correlation can be seen for an IDT geometry of λ = 6 µm. The measured values show a linear correlation between layer thickness and delay time. The slope, calculated by linear regression, represents the delay time offset produced by layer thickness difference. This slope is used to correct the thickness calculation. We define a reference thickness for every set of differential delay lines. As soon as there is a difference between measured layer thickness and reference thickness, the discrepancy is corrected by shifting the impulse response by t0. t0 represents the delay time offset. The impulse response is changed according to [14].
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Fig. 8. Comparison of phase velocity before and after etching and after tempering.
t0). In the spectral domain the multiplication of the signal U(f) with the factor e−i ωt0 results in an appropriate signal modulation. After this thickness correction for every pair of delay lines, the phase velocity can be recalculated. The comparison between the uncorrected phase velocity dispersion and the corrected phase velocity dispersion of the PVD process can be seen in Fig. 10. The phase velocity deviation is significantly reduced. The accuracy of the evaluation was improved by more than a factor 4 from Δvph,PVD = 17.3 m/s down to Δvph,PVD = 3.9 m/s. E. Material Constants Determination Simulations of the layered system were fitted to the corresponding measured phase velocities by optimizing the density ρ and the elastic constants c11 and c44 of the thin film. Therefore, different methods are suitable to simulate the free multilayer surface, such as Green’s function
u(t − t0 ) F e−i 2πt0 fU ( f ). (5) ↔
Thereby, the symbol F represents the Fourier transform. ↔ The time shift u(t) by the time t0 leads to a signal u(t −
Fig. 9. Correlation of layer thickness and measured delay time at λ = 6 µm.
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Fig. 10. Comparison of uncorrected and corrected phase velocities of the PVD SiO2 process.
Fig. 11. Comparison of simulated and measured phase velocities of SAWs on LiNbO3 128° rot. YX covered with PECVD and PVD SiO2.
or finite elements (FEM). We decided to use FEM for convenience. The constants of the substrate were taken from [11] and [12] and assumed to be accurate. Phase velocities of the layered system are simulated using FEM with randomly changing material constants of the film until they fit the corresponding measured phase velocities. The comparison between the measured phase velocities of LiNbO3 128° rot. YX covered with PECVD SiO2 and PVD SiO2, the FEM simulations can be seen in Fig. 11, and the comparison on LiNbO3 64° rot. YX is displayed in Fig. 12. The corresponding density ρ and elastic constants c11 and c44 of the PECVD and PVD SiO2 films are listed in Table II. The measured density was verified by weighing of the wafer before and after deposition of the SiO2 layer. The evaluated value using differential delay lines (Table II) is in excellent agreement with the value by weighing (ρ = 2174 ± 148 kg/m3). The rather large error using weighing arises from inaccuracy determining the volume of the film. Additionally, literature values of a PECVD and a PVD SiO2 were added to Table II. The method for how to determine the error of the material constants is explained in a different publication [15].
contact-free excitation and detection of the SAWs. Additionally, this technique provides a broadband wave excitation [17]. A. Film Preparation Silicon dioxide of about 4 µm thickness was deposited on a LiNbO3 substrate. Therefore, plasma enhanced chemical vapor deposition method (PECVD) was used due to its uniform growth on the substrate (see Section II-B). The layer thickness was determined using spectral ellipsometry. Additionally, a 30-nm-thick aluminum metal film was deposited on top of the SiO2 film. B. Excitation and Detection of SAWs A Nd:YAG laser at a wavelength of 1064 nm with 1 ns pulse duration is used to excite the SAWs. The laser pulse is sharply focused onto a line using a cylindrical
III. Laser Ultrasonic Measurement A different approach to determine the elastic constants of thin films uses laser induced SAWs. This technique has already been extensively used. An example is the determination of the dependency of the Young’s modulus of microcrystalline CVD diamond films on deposition conditions [6] or the observation of SAW dispersion generated by gradients in the mechanical and elastic properties of millimeter-thick microcrystalline diamond plates [16]. We used this approach to confirm the dispersion curve and thus the elastic constants determined by the differential delay-line technique. The advantage of using laser ultrasonics for determination of elastic constants is an efficient
Fig. 12. Comparison of simulated and measured phase velocities of SAWS on LiNbO3 64° rot. YX covered with PECVD and PVD SiO2.
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TABLE II. Comparison of Material Tensor Data of SiO2 Thin Films. c11 [GPa]
c44 [GPa]
ρ [kg/m3]
67.9 ± 1.4 84.5 ± 2.5 75.0 78.0
25.2 ± 0.6 31.7 ± 1.3 22.5 29.0
2143.6 ± 38.3 2347.4 ± 74.1 2185.0 2275.0
lens to achieve a well-defined propagation direction and to prevent diffraction [18]. The pump pulses are absorbed in the thin metal film within a depth of tens of nanometers. Thus, two counterpropagating strain pulses are launched by thermoelastic emission. These propagating surface perturbations were registered at different distances from the source. For this purpose, the continuous wave laser probe-beam deflection method is used [19] to monitor the transient slope of the SAW. A schematic picture of the measurement setup can be seen in Fig. 13. C. Measurement Results The propagation distance was scanned by 30 steps of 100 µm. This determines the spatial resolution of 3 mm. At every measured distance the Fourier transform of the SAW wave form was calculated, and for each Fourier component the phase ϕ was determined as a function of propagation distance x. Each of these phase curves was fitted by a linear function:
ϕ(ω) =
ω ⋅ x + ϕ0 (ω), (6) v ph (ω)
where ω = 2π∙f is the angular frequency and vph the phase velocity. The accuracy of the analysis can be improved significantly by using many steps. The evaluation method is explained in more detail in a previous publication [17]. We were measuring the Rayleigh wave mode, which propagates on LiNbO3 128° rot. YX. Additionally, we were measuring another Rayleigh wave mode by rotating the wafer by 90°. These two modes carry the information on the mechanical properties of the PECVD layer and were thus used for its evaluation. We were using the material constants determined in Section II-E to simulate
Fig. 13. Schematic diagram of a nanosecond pump-probe setup for generating SAW pulses with a pulsed laser and laser probing deflection with a position-sensitive detector.
Process
Reference
PECVD PVD PECVD PVD
This work This work [20] [21]
the phase velocities determined using laser ultrasound. The comparison between the measured phase velocities on LiNbO3 128° rot. YX covered with PECVD SiO2 and the simulated values can be seen in Fig. 14. The accuracy of the determined material constants using differential delay lines exceeds the accuracy using laser ultrasonics at the moment. However, the laser ultrasonics method has not been optimized yet. IV. Summary and Discussion We have investigated the acoustic properties of PECVD and PVD SiO2 thin films on LiNbO3 substrates by a differential delay line method. The density ρ and elastic constants c11 and c44 of the SiO2 films were determined by fitting FEM simulations to the measured phase velocities within an accuracy of at least ±4%. We found that the phase velocity of PVD SiO2 is consistently higher compared with PECVD SiO2. The PVD deposition process shows a significant variation in thickness across the wafer, in contrast to the PECVD process. Therefore, the delay lines used in the differential delay line evaluation method have different thicknesses, although they are close to each other. Thus, we introduced an experimental method to decrease the thickness variation using local etching of the wafer. This improved the accuracy of the phase velocity determination by a factor of 4. However, this method slightly changed the acoustic properties of the SiO2 layer.
Fig. 14. Comparison of measured and simulated phase velocities of LiNbO3 128° rot. YX and LiNbO3 128° rot. Y 90° rot. X covered with about 4 µm of PECVD SiO2.
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Therefore, we propose another method to improve the phase velocity determination accuracy, introducing a reference thickness for both delay lines used. We shift the corresponding time delays using the correlation between layer thickness variation and measured delay time. Thus we were able to reduce the scatter in phase velocity by at least a factor of 4. This method is favorable because it does not change the material characteristics. Thus the differential delay line evaluation method was successfully extended for application on layer stacks with nonuniform thicknesses. Within the accuracy of the determined phase velocities, the elastic constants of the SiO2 films do not vary across the wafer and between different wafers, if the same deposition method was applied. Additionally, we were measuring the SAW dispersion relation for two different propagation directions on LiNbO3 covered with PECVD SiO2 using laser-based method. Excellent agreement was achieved between the measurements of the phase velocities by the differential delay line method and the laser probe analysis. Acknowledgments The authors thank W. Gawlik and H. Öztürk for providing accurate wafer prober metrology, G. Scheinbacher for SiO2 layer deposition, and G. Grünauer for providing assistance in optimization. References [1] K. Hashimoto, M. Kadota, T. Nakao, M. Ueda, and M. Miura, “Recent development of temperature compensated SAW devices,” in Proc. IEEE Ultrasonics Symp., 2011, pp. 79–86. [2] T. E. Parker and H. Wichansky, “Temperature-compensated surface-acoustic-wave devices with SiO2 film overlays,” J. Appl. Phys., vol. 50, no. 3, pp. 1360–1369, 1979. [3] S. Matsuda, M. Miura, T. Matsuda, M. Ueda, Y. Satoh, and K. Hashimoto, “Investigation of SiO2 film properties for zero temperature coefficient of frequency SAW devices,” in IEEE Ultrasonics Symp., 2010, pp. 633–636. [4] V. I. Cherednick and M. Y. Dvoesherstov, “Surface and bulk acoustic waves in multilayer structures,” in Waves in Fluids and Solids, Rijeka, Croatia: InTech, 2011, pp. 69–102. [5] T. Malkow, I. Arce-Garcia, A. Kolitsch, D. Schneider, S. J. Bull, and T. F. Page, “Mechanical properties and characterisation of very thin CNx films synthesised by IBAD,” Diam. Relat. Mater., vol. 10, no. 12, pp. 2199–2211, 2001. [6] R. Kuschnereit, P. Hess, D. Albert, and W. Kulisch, “Density and elastic constants of hot-filament-deposited polycrystalline diamond films: Methane concentration dependence,” Thin Solid Films, vol. 312, pp. 66–72, 1998. [7] M. W. Elmiger, J. Henz, H. V. Kãnel, M. Ospelt, and P. Wachter, “Characterization of crystal surfaces, thin films and superlattices by brillouin scattering from surface acoustic modes,” Surf. Interface Anal., vol. 14, no. 1–2, pp. 18–22, 1989. [8] L. L. Chapelon, D. Neira, J. Torres, J. Vitiello, J. C. Royer, D. Barbier, F. Naudin, G. Tas, P. Mukundhan, and J. Clerico, “Measuring the Young’s modulus of ultralow-k materials with the nondestructive picosecond ultrasonic method,” J. Microelectron. Eng., vol. 83, no. 11–12, pp. 2346–2350, 2006. [9] R. Thomas, T. W. Johannes, W. Ruile, and R. Weigel, “Determination of phase velocity and attenuation of surface acoustic waves with improved accuracy,” in Proc. IEEE Ultrasonics Symp., 1998, pp. 277–282.
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[10] M. Knapp, P. Jäger, G. Grünauer, G. Scheinbacher, I. Bleyl, and L. M. Reindl, “Accurate determination of thin film properties using SAW differential delay lines,” in IEEE Int. Ultrasonics Symp., 2013, pp. 1704–1707. [11] G. Kovacs, M. Anhorn, H. E. Engan, G. Visintini, and C. C. W. Ruppel, “Improved material constants for LiNbO3 and LiTaO3,” in IEEE Ultrasonics Symp. Proc., 1990, pp. 435–438. [12] R. T. Smith and F. S. Welsh, “Temperature dependence of the elastic, piezoelectric and dielectric constants of lithium tantalate and lithium niobate,” J. Appl. Phys., vol. 42, no. 6, pp. 2219–2226, 1971. [13] G. Grünauer, M. Mayer, M. Knapp, P. Jäger, T. Ebner, K. Wagner, and H. J. Pesch, “Derivation of accurate tensor data of materials in SAW devices by solving a parameter identification problem using an enhanced eigenvalue analysis of an infinite array model,” in IEEE Int. Ultrasonics Symp., 2013, pp. 1700–1703. [14] H. Marko, Methoden der Systemtheorie. Berlin, Germany: Springer Verlag, 1977. [15] M. Knapp, P. Jäger, W. Ruile, I. Bleyl, and L. M. Reindl, “A refined method to determine the elastic constants of SiO2 thin films,” in IEEE Ultrasonics Symp., 2014, pp. 277–280. [16] G. Lehmann, M. Schreck, L. Hou, J. Lambers, and P. Hess, “Dispersion of surface acoustic waves in polycrystalline diamond plates,” Diam. Relat. Mater., vol. 10, no. 3–7, pp. 686–692, 2001. [17] Z. H. Shen, A. M. Lomonosov, P. Hess, M. Fischer, S. Gsell, and M. Schreck, “Multimode photoacoustic method for the evaluation of mechanical properties of heteroepitaxial diamond layers,” J. Appl. Phys., vol. 108, no. 8, art. no. 083524, 2010. [18] P. Hess, A. M. Lomonosov, and A. P. Mayer, “Laser-based linear and nonlinear guided elastic waves at surfaces (2d) and wedges (1d),” Ultrasonics, vol. 54, no. 1, pp. 39–55, 2014. [19] H. Coufal, K. Meyer, R. K. Grygier, P. Hess, and A. Neubrand, “Precision measurement of the surface acoustic wave velocity on silicon single crystals using optical excitation and detection,” J. Acoust. Soc. Am., vol. 95, no. 2, p. 1158–1160, 1994. [20] M. Tomar, V. Gupta, and K. Shreenivas, “Temperature coefficient of elastic constants of SiO2 over-layer on LiNbO3 for temperature stable SAW device,” J. Phys. D, vol. 36, no. 15, pp. 1773–1776, 2003. [21] F. S. Hickernell, H. D. Knuth, R. C. Dablemont, and T. S. Hickernell, “The surface acoustic wave propagation characteristics of 64° Y-X LiNbO3 and 36° Y-X LiTaO3 substrates with thin-film SiO2,” in IEEE Ultrasonics Symp., 1995, pp. 345–348.
Matthias Knapp studied physics at the Ruprecht-Karls University of Heidelberg, Germany, where he received his Dipl.-Phys. degree in 2011. Since then, he has been pursuing his Ph.D. degree, studying the characterization of functional layers in Surface Acoustic Wave (SAW) devices as an external Ph.D. student at the Laboratory for Electrical Instrumentation of Prof. L. M. Reindl in the Department of Microsystems Engineering (IMTEK), University of Freiburg, Germany. Since 2012 he has been an external employee at the EPCOS AG, which is a Group Company of TDK Corporation since 2008, in Munich, Germany, as a member in the Research and Development (R&D) Department. His primary research topics are signal processing and the characterization of layered systems for SAW devices. Matthias Knapp is a member of the IEEE.
Alexey M. Lomonosov studied at the Physics Faculty of Moscow State University, where he received his Master degree in 1984. He received the degree of Dr. Rer. Nat. at the University of Heidelberg, Germany, in 2002. In 1984 he joined the Prokhorov General Physics Institute of the Russian Academy of Sciences in Moscow, where he currently holds a position as Senior Research Fellow. His main field of research is laser physics, in particular laser ultrasound, which he pursues at his home institution and during research stays at the University of Heidelberg, Germany, the University of Le Mans, France, as a visiting professor at Nanjing University of Science and Technology, China, and at the University of Le Mans, France. He developed
knapp et al.: accurate characterization of sio2 thin films using surface acoustic waves methods to demonstrate the propagation of solitary SAW pulses, the investigation of brittle fracture of solids by means of strongly nonlinear pulses and nondestructive evaluation of residual stresses in metals.
Paul Warkentin, born in 1983, studied electrical engineering at Hochschule Offenburg, University of Applied Sciences. He received his Bachelor of Engineering degree in mechatronics in 2009 and his Master of Engineering degree in electrical engineering in 2012. In the same year, he took up a position as assistant at Hochschule Offenburg, where he worked in research and teaching projects in the areas of robotics, vehicle dynamics, and laser ultrasound.
Philipp M. Jäger, born in 1980, studied engineering physics at the University of Applied Sciences Munich, Germany. He received his Dipl. Ing. (FH) degree in 2010. After graduating he joined the R&D department of the Systems, Acoustics, Waves Business Group of EPCOS AG, a TDK Group Company. Since that time he has been involved in material research for acoustic wave devices and the development and optimization of these.
Werner Ruile received the Dipl. Phys. degree from the Ludwig-Maximilians University, Munich, Germany, in 1984. He received the Dr. Ing. degree of the Technical University Munich, Germany, in 1994. He was with the Corporate Research and Development, Siemens AG, Munich from 1983 until 2000, where he developed the simulation tools for SAW devices based on the P-Matrix. He joined the SAW research and development activities of the EPCOS AG in 2000, which is a Group Company of TDK Corporation since 2008, and worked on power durability, loss reduction, temperature compensation and trimming techniques. He currently focuses on nonlinearities in SAW filters.
Hans-Peter Kirschner was born 1966 in Rosenheim, Germany. He received his first degree of electronic technician in telecommunication at Deutsche Telekom in 1987 and his second degree of Dipl.-Ing. in electrical engineering at Fachhochschule Lands- hut, Germany, in 1993. Since then he was employed at EPCOS AG Munich, Germany, which is a Group Company of TDK Corporation since 2008, where he took part in the development of various processes for production of SAW filter for instance anodic passivation and frequency trimming. His primary interests are material modifications and precision corrective etching of wafer surface with ion beam and gas cluster ion beam technology.
Matthias Honal, born 1970 in Munich, Germany, received the Dipl. Min. degree from LudwigMaximilians University (LMU), Munich Germany in 1994 and the Dr. Rer. Nat. degree from the Federal Institute of Technology (ETH) Zurich, Switzerland in 1997, both in the field of crystallography. In 1999 he was a member of the micro acoustics group of the Siemens Corporate Technology Department, Munich, Germany, where he was engaged in research and development on SAW
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devices mainly concentrating on substrate materials at high temperatures. After several academic positions at Australian National University (ANU) Canberra, Australia, LMU Munich, Germany and University of Augsburg, Germany in 2000 to 2003 he joined the materials analytics group of Siemens Corporate Technology Department, Munich. Since early 2010 he has been employed at EPCOS AG, Munich, Germany, which is a Group Company of TDK Corporation since 2008, where his primary research topics are on improvement of materials and processing technologies for SAW devices.
Ingo Bleyl was born in 1968 in Kulmbach, Germany. He received his diploma degree in Experimental Physics in 1995 and his Ph.D. in 1998 from the University Bayreuth, Germany. In 1999 he joined EPCOS AG, which has been a Group Company of TDK Corporation since 2008. Within the Systems, Acoustics, Waves Business Group, he is involved in material research and development of surface acoustic wave devices.
Andreas P. Mayer received his Dipl. Phys. degree in 1983 and his Dr. Rer. Nat. degree in 1985 at the University of Münster, Germany. After research work at the universities of Münster, Perugia, Edinburgh, and UC–Irvine on lattice dynamics, surface physics, and nonlinear optics, he was an assistant and Privatdozent at the Physics Department of the University of Regensburg, Germany (1991–2001). In 2002, he joint Siemens VDO as a development engineer and in 2008 he became a professor at HS Offenburg, University of Applied Sciences. His main research interests are in microacoustics, especially nonlinear effects on wave propagation. Andreas Mayer is a member of IEEE.
Leonhard M. Reindl received the Dipl. Phys. degree from the Technical University of Munich, Germany in 1985 and the Dr. Sc. Techn. degree from the University of Technology Vienna, Austria, in 1997. From 1985 to 1999 he was a member of the microacoustics group of the Siemens Corporate Technology department, Munich, Germany, where he was engaged in research and development on SAW convolvers, dispersive, and reflective delay lines. His primary interest was in the development and application of SAW ID-tag and wireless passive SAW sensor systems. In winter 1998–1999 and in summer 2000, he was a guest professor for spread spectrum technologies and sensor techniques at the University of Linz, Austria. From 1999-2003 he was university lecturer for communication and microwave techniques at the Institute of Electrical Information Technology, Clausthal University of Technology. In May 2003 he accepted a full professor position at the laboratory for electrical instrumentation at the Department of Microsystems Engineering (IMTEK), University of Freiburg.
